1. Field of the Invention
The present invention relates to high-frequency oscillation apparatuses, radio apparatuses, and radars which include an oscillator for generating an oscillation signal in ultra high frequency bands such as the microwave band and the millimeter wave band.
2. Description of the Related Art
In general, frequency modulated, continuous wave radars (FM-CW radars) FM-modulate an oscillation frequency in a predetermined frequency range, continuously output a signal having a specific frequency, and receive a wave reflected from an object to observe the object. In the FM-CW radars, the oscillation frequency is modulated with the use of triangular waves having a frequency of about 100 MHz or higher, which are superior in linearity.
To generate triangular waves, which are superior in linearity, the following methods can be examined in general.
(1) Using a voltage-controlled oscillator VCO (hereinafter just called a “VCO”) having a constant frequency-modulation sensitivity.
(2) Deforming a modulation signal in advance in the direction reverse to that of a modulation sensitivity characteristic so as to have a constant frequency-modulation sensitivity to obtain linearity.
(3) Forming a closed-loop circuit which monitors the oscillation frequency continuously, and applies compensation if the frequency is shifted.
In the method shown in (1), it is necessary to use a VCO having a superior modulation characteristic. Currently, it is very difficult to make such a VCO. If it is made, it is very expensive.
In the method shown in (2), successful linearity can be obtained relatively easy by using a low-frequency circuit. Since an open-loop circuit is compensated, the characteristic of the circuit may be shifted due to a change in the characteristic of the VCO, so that linearity is not obtained.
As a representative way to implement the method shown in (3), a phase-locked loop (PLL) system has been used. With this system, triangular waves, superior in linearity, are obtained, and even if the characteristic of the VCO is changed in time, it can be compensated. The PLL system requires, however, a reference oscillator, such as a crystal oscillator, and a scaler. Its structure is complicated and it is very expensive.
As a high-frequency oscillation apparatus in which the structure of the PLL is simplified and the oscillation center frequency is made stable, there is an apparatus (such as that disclosed in Japanese Unexamined Patent Application Publication No. 2002-223169) which includes a resonator coupled with a line transferring an oscillation signal sent from a VCO, a detector coupled with the resonator, for detecting the resonant-energy level output from the resonator, and a CPU for generating and outputting a VCO control voltage signal according to the output of the detector, and in which the resonant frequency of the resonator is set so as to match the oscillation center frequency of the oscillator.
A high-frequency oscillation apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-223169 uses an FM-CW frequency modulation signal to measure changes of a resonant-energy level (passing-through-signal level) in time and detects the maximum value, that is, the resonant frequency of a resonator. Then, the apparatus detects the shift of the center frequency by the timing when the maximum value of the resonant-energy level appears to obtain the stable oscillation center frequency.
In such a high-frequency oscillation apparatus, it is easy to detect the shift of the center frequency and compensate for the shift. When each modulation waveform is compensated, however, since the oscillation center frequency matches the resonant frequency, that is, the resonant-energy level has the maximum value (peak) at the oscillation center frequency, the frequency of a modulated signal in which the oscillation center frequency is modulated by a desired frequency does not uniquely correspond to the output of the detector for detecting the resonant-energy level. Therefore, signal processing after demodulation, until the setting of a control voltage signal becomes complicated. Since the oscillation center frequency is set to the frequency where the resonant energy has a peak, changes in the resonant-energy level, corresponding to changes in frequency are very small around the peak, compared with other portions away from the peak. Therefore, it is difficult to correctly obtain the oscillation frequency from the resonant-energy level near the resonant frequency.